1. Field of the Invention
The present invention relates to a micro-electro-mechanical sensor with force feedback loop.
2. Description of the Related Art
As is known, the use of micro-electro-mechanical systems or MEMS is increasingly widespread in various sectors of technology and has yielded encouraging results especially in the construction of inertial sensors, micro-integrated gyroscopes, and electromechanical oscillators for a wide range of applications.
MEMS systems of this type are usually based upon micro-electro-mechanical structures comprising at least one mass, which is connected to a fixed body (stator) by means of springs and is movable with respect to the stator according to predetermined degrees of freedom. The movable mass and the stator are capacitively coupled by means of a plurality of respective comb-fingered electrodes set facing one another, so as to form capacitors. The movement of the movable mass with respect to the stator, for example on account of an external stress, modifies the capacitance of the capacitors. From this it is possible to trace back to the relative displacement of the movable mass with respect to the fixed body and hence to the applied force. Instead, by supplying appropriate biasing voltages, it is possible to apply an electrostatic force to the movable mass to set it in motion. Furthermore, in order to obtain electromechanical oscillators, the frequency response of the inertial MEMS structures is exploited, which typically is of a second-order low-pass type. By way of example, FIGS. 1 and 2 show the trend of the module and of the phase of the transfer function between the force applied to the movable mass and its displacement with respect to the stator, in an inertial MEMS structure.
Reading of many types of MEMS systems, such as, for example, inertial sensors, sensors of other types, or gyroscopes, is performed using a force feedback loop. In practice, the capacitive unbalancing due to a displacement of the movable mass is read and, by means of the force feedback loop, electrostatic forces tending to eliminate the displacement, on the basis of the capacitive unbalancing detected, are applied. The amplitude of the electrostatic forces required is indicative of the external stress acting on the movable mass and can be estimated on the basis of the signals present in the force feedback loop.
Known solutions generally envisage the use of sigma-delta modulators for transduction of the capacitive unbalancing in the force feedback loops. Reading devices of this type are undoubtedly precise and effective, in so far as the sigma-delta modulators are reliable and have a good speed of response, especially if they are made directly of hardware/firmware. However, the bitstream supplied by the sigma-delta modulator must be filtered, decimated and further processed and, hence, it is necessary to envisage purposely designed stages for each operation. Hence, currently available reading devices for MEMS are complex to produce, cumbersome and, in practice, costly.